The invention relates to radiation-curable, optical fiber coating systems comprising an inner and an outer primary coating compositions. The invention also relates to coated optical fibers and optical fiber assemblies. More particularly, the invention relates to a radiation-curable, optical fiber coating system that provides improved strip cleanliness and optical fibers coated with the coating system, ribbon assemblies comprising such coated optical fibers and methods of making and forming the same.
Optical fiber coating systems commonly comprise two coating compositions. The first coating composition contacts the glass surface and is called the inner primary coating. The second coating composition is designed to overlay the inner primary coating and is called the outer primary coating.
The inner primary coating is usually a soft coating having a low glass transition temperature (hereinafter xe2x80x9cTgxe2x80x9d), to provide resistance to microbending. Microbending can lead to attenuation of the signal transmission capability of the coated optical glass fiber and is therefore undesirable. The outer primary coating is typically a harder coating providing desired resistance to handling forces, such as those encountered when the coated fiber is cabled.
For the purpose of multi-channel transmission, optical fiber assemblies containing a plurality of coated optical fibers have been used. Examples of optical fiber assemblies include ribbon assemblies and cables. A typical optical fiber assembly is made of a plurality of coated optical fibers which are bonded together in a matrix material. For example, the matrix material can encase the optical fibers, or the matrix material can edge-bond the optical fibers together.
Optical fiber assemblies provide a modular design which simplifies the installation and maintenance of optical fibers by eliminating the need to handle individual optical fibers.
Coated optical fibers for use in optical fiber assemblies are usually coated with an outer colored layer, called an ink coating, or alternatively a colorant is added to the outer primary coating to facilitate identification of the individual coated optical glass fibers. Such ink coatings and colored outer primary coatings are well known in the art. Thus, the matrix material which binds the coated optical fibers together contacts the outer ink layer if present, or the colored outer primary coating.
When a single optical fiber of the assembly is to be fusion connected with another optical fiber, or with a connector, an end part of the matrix layer is required to be stripped away from the optical fiber. A common method for practicing ribbon stripping at a terminus of the ribbon assembly is to use a heated stripping tool. Such a tool consists of two plates provided with heating means for heating the plates to about 90 to about 120xc2x0 C. An end section of the ribbon assembly is pinched between the two heated plates and the heat of the tool softens the matrix material and the primary coatings prior to and during the stripping procedure.
Ideally, the primary coatings on the coated optical fibers, and the ink coating if present, are removed simultaneously with the matrix material to provide bare portions on the surface of the optical fibers (hereinafter referred to as xe2x80x9cribbon strippingxe2x80x9d). In ribbon stripping, the matrix material, primary coatings, and ink coating, are desirably removed as a cohesive unit to provide a clean, bare optical glass fiber which is substantially free of residue. Any residue can interfere with the optical glass fiber ribbon mass fusion splicing operation, and therefore is presently removed by wiping with a solvent prior to splicing. However, the solvent wipe can cause abrasion sites on the bare optical fiber, thus compromising the integrity of the connection. Many attempts have been made to increase the strip cleanliness of the ribbon assemblies by adding adhesion reducing additives to the inner primary coating which results in systems with little improvement in the strip cleanliness or system with insufficient adhesion. The ability to produce ribbon assemblies that can be stripped to provide clean, residue-free, bare optical glass fibers without unduly sacrificing other desirable or required properties of the primary coatings continues to challenge the industry.
There are many test methods,which may be used to determine the performance of a ribbon assembly during ribbon stripping. An example of a suitable test method for determining the stripping performance of a ribbon is disclosed in the article by Mills, G., xe2x80x9cTesting of 4- and 8-fiber ribbon strippabilityxe2x80x9d, 472 International Wire and Cable Symposium Proceedings (1992), the complete disclosure of which is incorporated herein by reference.
Many attempts have been made to understand the problems associated with ribbon stripping and to find a solution to increase ribbon stripping performance. The following publications attempt to explain and solve the problems associated with ribbon stripping: K. W. Jackson, et. al., xe2x80x9cThe Effect of Fiber Ribbon Component Materials on Mechanical and Environmental Performancexe2x80x9d, 28 International Wire and Symposium Proceedings (1993); H. C. Chandon, et. al., xe2x80x9cFiber Protective Design for Evolving Telecommunication Applicationsxe2x80x9d, International Wire and Symposium Proceedings (1992); J. R. Toler, et. al., xe2x80x9cFactors Affecting Mechanical Stripping of Polymer Coatings From Optical Fibersxe2x80x9d, International Wire and Cable Symposium Proceedings (1989); and W. Griffloen, xe2x80x9cStrippability of Optical Fibersxe2x80x9d, EFOC and N, Eleventh Annual Conference, Hague (1993).
The ability of a ribbon assembly to ribbon strip cleanly so as to provide bare optical glass fibers that are substantially free of residue was heretofore unpredictable and the factors affecting ribbon stripping not fully understood. Accordingly, there is a need for an optical fiber, radiation-curable coating composition system that improves the strippability of optical fiber ribbons.
It is an objective of the present invention to provide a radiation-curable, optical fiber primary coating system comprising an inner primary coating composition and a, colored or non-colored, outer primary coating composition that imparts improved ribbon stripping to a ribbon assembly, when incorporated therein.
It is another objective of the present invention to provide a coated optical fiber having a coating such that when it is incorporated into a ribbon assembly the ribbon assembly achieves better strip cleanliness.
It is another objective of the present invention to provide a ribbon assembly having improved ribbon stripping capabilities.
It is still a further objective of the present invention to provide a method of preparing a radiation-curable, optical fiber coating system comprising an inner primary coating composition and an outer primary coating composition that imparts improved ribbon stripping to a ribbon assembly, when incorporated therein.
Surprisingly, the above objects and other objects are and have been obtained by the following. The present invention provides a radiation-curable, optical fiber primary coating system having; i) a radiation-curable inner primary coating composition comprising at least one strip enhancing component wherein said composition, after cure, is capable of sufficiently adhering to an optical fiber so as to prevent delamination in the presence of moisture and during handling, and ii) a radiation-curable outer primary coating composition that, upon cure, has a secant modulus of at least 1000 MPa (when measured on Mylar). Suitable strip enhancing components including, for example:
a. an oligomer comprising at least one strip agent moiety and/or a composite oligomer comprising at least one glass coupling moiety, at least one slip agent moiety, and at least one radiation-curable moiety capable of polymerizing under the influence of radiation;
b. a soluble wax that is soluble in said inner primary coating composition and/or a solid lubricant;
c. a radiation-curable silicone oligomer and/or a silicone compound which may be either non-radiation-curable or radiation-curable and/or mixtures thereof;
d. a silicone compound containing at least one radiation-curable functional group bound near a terminus of said compound;
e. a fluorinated component selected from the group consisting of a radiation-curable fluorinated oligomer, a radiation-curable fluorinated monomer, a non-radiation curable fluorinated compound or mixtures thereof;
f. a radiation-curable oligomer comprising at least one terminal linear moiety and/or at least one substantially linear radiation-curable oligomer;
g. a low-urethane oligomer wherein the calculated molecular weight concentration of said urethane groups in said oligomer is about 4% by weight or less, based on the total calculated molecular weight of the oligomer;
h. a radiation-curable oligomer having a polymeric backbone that has a molecular weight of at least about 2000, preferably more than 3100;
i. a radiation-curable oligomer and/or monomer diluent having a high aromatic content; and/or
j. other slip enhancing additives and/or components discussed in applicants concurrently pending U.S. application Ser. No. 09/035,771, the entire disclosure of which is hereby incorporated by reference.
Also provided by the present invention is a coated optical fiber coated with a radiation-curable, optical fiber primary coating system discussed herein, a ribbon assembly comprising: a plurality of optical fibers, at least one optical fiber coated with a radiation-curable, optical fiber primary coating system discussed herein, and optionally an ink coating; and a matrix material bonding said plurality of coated optical fibers together; and processes for making such coated optical fibers and ribbon assemblies.
Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings.
The present invention is direct to radiation-curable, optical fiber primary coating systems (optical fibers coated with a primary coating system, and ribbon assemblies comprising such coated optical fibers) that provide improved ribbon stripping (i.e., when incorporated into a ribbon assembly enable the cured coatings, matrix materials and optional inks coating materials to strip relatively cleanly from the optical fiber). The primary coating systems of the present invention combines a radiation-curable inner primary coating having a slip enhancing component with a relatively high secant modulus outer primary coating to achieve improved ribbon stripping. The present invention provides a radiation-curable, optical fiber coating system comprising a radiation-curable inner primary coating composition and a radiation-curable outer primary coating composition. The coating compositions according to the present invention include those formulated from (A) an oligomer (often referred to as a pre-polymer) system, (B) a monomer diluent system, (C) an optional photoinitiator system, and (D) additives. Background teachings on how to formulate and apply radiation-curable compositions for fiber optic materials can be found in, for example, U.S. Pat. Nos. 5,384,342; 5,456,984; 5,596,669; 5,336,563; 5,093,386; 4,716,209; 4,624,994; 4,572,610; and 4,472,019, which are hereby incorporated in their entirety by reference.
For this invention, xe2x80x9cpre-mixture ingredientsxe2x80x9d means that when formulating a radiation-curable composition from its ingredients, some interaction or reaction of the ingredients is possible in some cases after mixing. However, pre-mixture ingredient refers to the identity of the ingredient before any such interaction or reaction of the ingredients might occur after mixing.
Also, for this invention, xe2x80x9c(meth)acrylatexe2x80x9d means acrylate, methacrylate, or a mixture thereof. For this invention, xe2x80x9cpre-polymerxe2x80x9d and xe2x80x9coligomerxe2x80x9d have equivalent meaning.
(A) The Oligomer System
In this invention, the pre-polymer or oligomer system comprises one or more radiation-curable oligomers. In general, an oligomer system is first prepared, optionally in the presence of a monomer diluent. Then, the oligomer formulation is further formulated by mixing with other ingredients such as monomer diluents, photoinitiator, and additives. If multiple oligomers are desired, individual oligomers can be synthesized separately, and then mixed, or they can be synthesized together in a single, one-pot synthesis. In either case, the synthesis of oligomers often produces a statistical distribution of different types of oligomers which can be represented by idealized structures.
If some ingredients from the monomer diluent system (see below) are present during oligomer synthesis, they are not considered part of the pre-polymer system because, in general, monomer diluent does not react substantially during oligomer preparation and merely functions as a solvent for oligomer synthesis. In general, a monomer diluent can be distinguished from an oligomer because it will have a lower molecular weight than an oligomer, and will serve to decrease the viscosity of an oligomer. However, some monomer diluents can have repeating units such as repeating alkoxy units. However, for this invention, if the diluent functions to decrease the viscosity of the oligomer, then it is a called a diluent rather than an oligomer.
The amount of the oligomer system (A) can be, for example, about 10 wt. % to about 90 wt. %, and preferably, between about 20 wt. % to about 80 wt. %, and more preferably, about 30 wt. % to about 70 wt. %. Preferably, the oligomer amount is about 50 wt. %. If more than one oligomer is present, then the wt. % of each oligomer is added.
Radiation-curable oligomers can comprise one or more radiation-curable end groups and an oligomer backbone. The end-group provides a cure mechanism, whereas the backbone provides suitable mechanical properties upon cure. In addition, the oligomer can comprise one or more linking groups such as a urethane- or urea-containing moiety which further can improve the mechanical performance of cured compositions. The linking groups can link an oligomeric backbone moiety to the radiation-curable end-group, or link oligomeric backbone moieties to themselves. Hence, for example, radiation-curable oligomers can be prepared from three basic components (backbone, linking, and radiation-curable components) and can be represented by structures such as, for example:
Rxe2x80x94[Lxe2x80x94B]xxe2x80x94Lxe2x80x94R
where R is a radiation-curable group, L is a linking group, and B is a backbone moiety. The variable x indicates the number of backbone moieties per oligomer molecule. This value X can be controlled by, for example, control of the reaction stoichiometry during oligomer synthesis. Typically, X is designed to be 1. In this representation, L and B are difunctional moieties, but oligomers can also be prepared from tri- and higher functional L and B moieties to provide branching. In the present invention, branching points in the oligomer are preferably present, and preferably result from use of at least some tri-functional groups L. Then, for example, an oligomer can also be represented by:
(R)2xe2x80x94Lxe2x80x94Bxe2x80x94Lxe2x80x94(R)2
In particular, typical radiation-curable urethane acrylate oligomers according to the present invention are prepared from (i) at least one ingredient which reacts to provide the radiation-curable acrylate group R, (ii) at least one ingredient which reacts to provide the urethane linking group L, and (iii) at least one ingredient which reacts to provide the backbone B. Different urethane acrylate oligomer synthetic strategies are disclosed in, for example, U.S. Pat. No. 5,093,386, which is hereby incorporated by reference. Other synthetic methods, however, may be used to prepare equivalent structures. These methods may be adapted by methods known in the art to provide urea linkages, methacrylate linkages, and other common types of linkages found in radiation-curable oligomers.
The radiation-curable oligomer can cure by reaction of its radiation-curable groups, R, via a free-radical mechanism or by cationic mechanism. A free-radical cure, however, is preferred. Ethylenically unsaturated groups are preferred. Exemplary radiation-curable groups include (meth)acrylate, vinyl ether, vinyl, acrylamide, maleate, fumarate, and the like. The radiation-curable vinyl group can participate in thiol-ene or amine-ene cure. Most preferably, the radiation-curable group is an acrylate if fast cure speed is desired.
Preferably, the oligomer comprises at least two radiation-curable groups, and preferably, at least two ethylenically unsaturated groups. The oligomer, for example, can comprise two, three, or four radiation-curable groups which are all preferably ethylenically unsaturated groups. There is no strict upper limit on the number of radiation-curable groups per oligomer, but in general, the number of radiation-curable groups is less than 10, and preferably, less than 8.
The oligomer can comprise copolymeric structures including random and block copolymeric structures. Methods known in the art can be used to prepare such copolymeric structures. For example, backbone moieties can be copolymeric. Also, a one-pot synthesis of multiple oligomers can be executed with use of multiple backbone moieties. Using multiple backbone moieties can yield at least some block copolymeric oligomers in the pre-polymer system. Formulation design of copolymeric oligomers can result in a better balance of properties and provide synergistic effects, which usually is crucial for fiber optic materials. In addition, oligomer blends or mixtures can be used to balance properties and provide synergistic effects..
For processing reasons, it is important to control the oligomer system""s viscosity and flow behavior. For practical reasons, oligomers should be easy to remove from the reactors and flasks in which they are synthesized. If viscosity is too high, it will be difficult to process the oligomer system during formulation, even with some monomer diluent present.
If an oligomeric polyether diol is used, the polyether may include, for example, substantially non-crystalline polyethers. The oligomer may include polyethers comprising repeating units of one or more of the following monomer units:
xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94Oxe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94
xe2x80x94Oxe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94Oxe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94
xe2x80x94Oxe2x80x94CH2xe2x80x94CH(CH)3xe2x80x94CH2xe2x80x94CH2xe2x80x94
xe2x80x94Oxe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94CH2xe2x80x94CH2xe2x80x94.
xe2x80x94Oxe2x80x94CH(CH2CH3)xe2x80x94CH2xe2x80x94.
xe2x80x94Oxe2x80x94CH2xe2x80x94C(CH3)(CH3)xe2x80x94, and the like.
An example of a polyether polyol that can be used is the polymerization product of (i) tetrahydrofuran, or (ii) a mixture of 20 percent by weight of 3-methyltetrahydrofuran and 80 percent by weight of tetrahydrofuran, both of which have undergone a ring opening polymerization. This latter polyether copolymer contains both branched and non-branched oxyalkylene repeating units and is marketed as PTGL 1000 (Hodogaya Chemical Company of Japan). Another example of a polyether in this series which can be used is PTGL 2000 (Hodogaya Chemical Company). Butyleneoxy repeat units are preferred to impart flexibility to one oligomer in particular and the pre-polymer system in general.
If a polyolefin diol is used, the polyolefin is preferably a linear or branched hydrocarbon containing a plurality of hydroxyl end groups. Fully saturated, for example, hydrogenated hydrocarbons, are preferred because the long term stability of the cured coating increases as the degree of unsaturation decreases. Examples of hydrocarbon diols include, for example, hydroxyl-terminated, fully or partially hydrogenated 1,2-polybutadiene; 1,4- and 1,2-polybutadiene copolymers, 1,2-polybutadiene-ethylene or -propylene copolymers, polyisobutylene polyol; mixtures thereof, and the like.
Other suitable oligomers may include polyester oligomers, polycarbonates oligomers, mixtures of any of the aforementioned oligomer types and the like.
The linking group of the oligomer can be a urethane or urea group, and preferably is a urethane group. It is well-known in the art that urethane linkages can be formed by reaction of a polyfunctional isocyanate with a hydroxy compound including a hydroxy-containing backbone component or a hydroxy-containing radiation-curable component.
Polyfunctional isocyanates include diisocyanates, triisocyanates, and higher order polyisocyanates which can provide the linking group. As known in the art, isocyanate compounds can be trimerized to form isocyanurate compounds which can provide the linking group. Hence, polyisocyanate compounds can be oligomerized or polymerized to form higher order polyisocyanates comprising isocyanurate group. Isocyanurate compounds are a preferred example of how to provide a cyclic group having the capacity to hydrogen bond.
Generally, the compound providing a radiation-curable terminus to the oligomer contains a functional group which can polymerize under the influence of actinic radiation and a functional group which can react with the diisocyanate. Hydroxy functional ethylenically unsaturated monomers are preferred. More preferably, the hydroxy functional ethylenically unsaturated monomer contains acrylate, methacrylate, vinyl ether, maleate or fumarate functionality.
In the reaction between hydroxy group of the compound providing the terminus and isocyanate groups of compound providing the linking sites, it is preferred to employ a stoichiometric balance between hydroxy and isocyanate functionality and to maintain the reaction temperature of at least 25xc2x0 C. The hydroxy functionality should be substantially consumed. The hydroxy functional ethylenically unsaturated monomer attaches to the isocyanate via a urethane linkage. Monomers having (meth)acrylate functional groups include, for example, hydroxy functional (meth)acrylates such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, methacrylate analogs, and the like. Monomers having vinyl ether functional groups include, for example, 4-hydroxybutyl vinyl ether, and triethylene glycol monovinyl ether. Monomers having maleate functional groups include, for example, maleic acid and hydroxy functional maleates.
There is no particular limitation on the molecular weight of the oligomer, but the number average molecular weight of the oligomer in general can be less than about 25,000 g/mol, and preferably, less than about 10,000 g/mol, and more preferably, less than about 5,000 g/mol. Molecular weight is preferably greater than about 500 g/mol.
(B) The Monomer Diluent System
The compositions according to the invention also comprises a monomer, or reactive, diluent system which comprise at least one monomer diluent. The reactive diluent can be used to adjust the viscosity of the coating composition. Thus, the reactive diluent can be a low viscosity monomer containing at least one functional group capable of polymerization when exposed to actinic radiation.
The reactive diluent is preferably added in such an amount that the viscosity of the coating composition is in the range of about 1,000 to about 10,000 mPaxc2x7s.
Suitable amounts of the reactive diluent have been found to be about 10 wt % to about 90 wt %, and more preferably about 20 wt. % to about 80 wt. %, and more preferably, about 30 wt. % to about 70 wt. %.
A monomer diluent preferably has a molecular weight of not more than about 550 or a viscosity at room temperature of not more than about 300 mPaxc2x7s (measured as 100% diluent).
The radiation-curable functional group present on the reactive diluent may be of the same nature as that used in the radiation-curable oligomer. Preferably, the radiation-curable functional group present in the reactive diluent is capable of copolymerizing with the radiation-curable functional group present on the radiation-curable oligomer. Ethylenic unsaturation is preferred. In particular, acrylate unsaturation is preferred.
Preferably, the reactive diluent system comprises a monomer or monomers having an acrylate or vinyl ether functionality and an C4-C20 alkyl or polyether moiety. Examples of such reactive diluents include hexylacrylate, 2-ethylhexylacrylate, isobomylacrylate, decylacrylate, laurylacrylate, stearylacrylate, ethoxyethoxy-ethylacrylate, laurylvinylether, 2-ethylhexylvinyl ether, N-vinyl formamide, isodecyl acrylate, isooctyl acrylate, vinyl-caprolactam, N-vinylpyrrolidone and the like.
Another type of reactive diluent is a compound comprising an aromatic group. Examples of diluents having an aromatic group include:
ethyleneglycolphenyletheracrylate,
polyethyleneglycolphenyletheracrylate,
polypropyleneglycolphenyletheracrylate, and
alkyl-substituted phenyl derivatives of the above monomers, such as
polyethyleneglycolnonylphenyletheracrylate.
Furthermore, a reactive diluent can contain two groups capable of polymerization using actinic radiation. A diluent having three or more of such reactive groups can be present as well. Examples of such monomers include:
C2-C18 hydrocarbondioldiacrylates,
C4-C18 hydrocarbondivinylethers,
C3-C18 hydrocarbontrioltriacrylates,
the polyether analogs thereof, and
the like, such as
1,6-hexanedioldiacrylate,
trimethylolpropanetriacrylate,
hexanedioldivinylether,
triethyleneglycoldiacrylate,
pentaeritritoltriacrylate,
tripropyleneglycol diacrylate
alkoxylated bisphenol A diacrylate.
Preferably, the oligomer and the reactive diluent each contain an acrylate group as a radiation-curable group.
(C) The Optional Photoinitiator System
The composition may optionally further comprise at least one photoinitiator. Photoinitiator is required for a fast UV cure but may be omitted for electron beam cure. Conventional photoinitiators can be used. Examples include benzophenones, acetophenone derivatives, such as alpha-hydroxyalkylphenylketones, benzoin alkyl ethers and benzil ketals, monoacylphosphine oxides, and bisacylphosphine oxides. A preferred photoinitiator is 1-hydroxycyclohexylphenylketone (Irgacure 184, Ciba Geigy).
Often mixtures of photoinitiators provide a suitable balance of properties.
The amount of photoinitiator system is not particularly limited but will be effective to provide fast cure speed, reasonable cost, good surface and through cure, and lack of yellowing upon aging. Typical amounts can be, for example, about 0.3 wt. % to about 10 wt. %, and preferably, about 1 wt. % to about 5 wt. %.
(D) Additives
Conventional additives can be used in effective amounts. For example, additives such as stabilizers to prevent gellation, UV screening compounds, leveling agents, polymerization inhibitors, light stabilizers, chain transfer agents, colorants including pigments and dyes, plasticizers, fillers, wetting improvers, preservatives, and the like can be used. Other polymers and oligomers can be added to the compositions. Moisture content in the coatings is preferably minimized.
Preferred inner primary coating compositions of the primary coating system of the present invention comprise in place of or in addition to the above-noted components a sufficient amount of one or more slip enhancing components such that after cure the composition has in addition to:
i) a glass transition temperature below 0xc2x0 C., preferably below xe2x88x9210xc2x0 C., more preferably below xe2x88x9220xc2x0 C.; and
ii) sufficient adhesion to said glass fiber to prevent delamination in the presence of moisture, preferably, an adhesion of at least 5 g/in when conditioned at 95% (RH);
a fiber friction value of less than 40 g/mm, preferably less than 30 g/mm, more preferably less than 20 g/mm, most preferably greater than 10 g/mm and a crack propagation of greater than 0.7 mm at 90xc2x0 C., preferably greater than 1.0 mm, more preferably greater than 1.5 mm, and most preferably, greater than 2 mm at the desired/design ribbon stripping temperature, of for instance 90xc2x0 C.
Preferred outer primary coating compositions of the primary coating system of the present invention which may be colored or non-colored include compositions generally formulated from components as set forth herein that have, after cure, a secant modulus of greater than 1000 MPa, preferably greater than 1050 MPa, and more preferably greater than 1100 MPa at 23xc2x0 C. and a glass transition temperature of above 40xc2x0 C., preferably above 50xc2x0 C.
(E) Slip Enhancing Component
1) Composite Oligomer
A suitable slip enhancing component of the present invention includes a composite oligomer that can be used to adjust the fiber friction between the inner primary coating and the surface of the optical glass fiber. The composite oligomer comprises at least one glass coupling moiety, at least one slip agent moiety, and/or at least one radiation-curable moiety. Preferably, the composite oligomer will comprise at least two different types of these moieties, for example, at least one a slip agent moiety and at least one radiation-curable moiety, more preferably the composite oligomer will comprise at least one of each of these three types of moieties. Preferably, the various moieties of the composite oligomer are covalently bonded together. Linkage of these moieties can be direct so that there are no intermediate linking groups between the oligomer and the moiety. Alternatively, however, the linkage can be indirect by using intermediate linking groups.
A variety of glass coupling, slip agent, and radiation-curable moieties are known in the art. The present invention can be practiced with use of various embodiments using different combinations of these moieties to produce a composite oligomer. A person skilled in the art will easily be able to prepare combinations of these various moieties from the present disclosure and general knowledge in the art.
Radiation-curing can occur by reaction of the composite oligomer""s radiation-curable moieties with themselves and/or with radiation-curable moieties bound to other components of a formulation. In general, curing of the composite oligomer occurs in concert with other radiation-curable components. The inner primary compositions comprising a composite oligomer are preferably directed to reactions wherein the radiation-curable moiety reacts not with the glass coupling or slip agent moieties. For example, although the glass coupling moiety will be reactive, and is often sensitive to hydrolysis and condensation reactions, these types of reactions are not the preferred cure mechanism.
The molecular weight of the composite oligomer is not limited. In general, however, the molecular weight of the composite oligomer in its uncured state is usually between about 200 and about 10,000, preferably between about 500 and about 5,000.
There is no particular limitation on the molecular architecture of the composite oligomer, although linear or substantially linear oligomeric structures are preferred over other useful non-linear, cyclic, or branched structures. A substantially linear structure means that there is a single, dominant linear oligomeric backbone which is xe2x80x9ccappedxe2x80x9d at the two ends of the backbone. The amount of branching units in the backbone is generally less than about 10 mole %, and preferably, less than about 5 mole %. The linear backbone may contain one or more types of repeat units, although preferably, one major type of repeat unit is used. Nevertheless, block or random copolymeric structures can be used if necessary. With a substantially linear backbone, the number of branch points in the backbone will be kept to a minimum, and preferably, will not be used. Synthetic simplicity in the oligomer structure is preferred to the extent that cost-performance can be achieved.
The term xe2x80x9cglass coupling moietyxe2x80x9d is understood to mean a functional group which is known or has the ability to improve adhesion to an inorganic surface and in particular, a glass surface. Preferably, the glass coupling moiety includes groups that are known to covalently bond with an inorganic material, particularly as a result of a hydrolysis and/or condensation reaction. Suitable glass coupling moieties may include those derived from conventional coupling agents including conventional silane coupling agents disclosed in E. P. Plueddemann""s Silane Coupling Agents, Plenum Press (1982), the complete disclosure of which is hereby incorporated by reference; non-silane coupling agents, for example, chromium, orthosilicate, inorganic ester, titanium, and zirconium systems. Although the present invention is preferably practiced with use of silane glass coupling moieties, the invention is not so restricted, and a person skilled in the art is enabled by the present disclosure to use these other systems as well.
In the present invention, the glass coupling moieties may be derived from these conventional coupling agents which are covalently incorporated into the composite oligomer in a manner which preserves their coupling function to the inorganic surface. In a preferred embodiment, for example, the organic component of a conventional coupling agent is linked covalently, either directly or indirectly, with the composite oligomer which additionally comprises slip agent and radiation-curable moieties. After this linkage, the glass coupling moiety will still have its inorganic component effective for bonding with the inorganic surface or at the inorganic-organic interface. However, the invention is not so limited, and the glass coupling moiety is not necessarily linked to the composite oligomer by reaction of the organic functional group of a conventional coupling agent.
Silane coupling moieties are preferred. Silane coupling moieties having at least one hydrolyzable xe2x80x9cxe2x80x94Sixe2x80x94Oxe2x80x94Rxe2x80x9d linkage are particularly preferred. Even more preferred are silane coupling moieties represented by the following formula:
xe2x80x94Si(OR)3
where R represents a C1-C4 alkyl group, preferably methyl or ethyl, which imparts at least some hydrolyzability to the silane.
Common organic functionalities of the silane coupling agents include, for example, amino, epoxy, vinyl, (meth)acryloxy, isocyanato, mercapto, polysulfide, and ureido. Using synthetic methods known in the art, the organic functionality can be reacted with the oligomer to yield a covalent linkage between the glass coupling moiety and the oligomer. In a preferred embodiment, for example, mercaptopropyl silane is linked with an oligomer containing an isocyanate group to form a thiourethane adduct between the mercapto group and the isocyanate group. Although a strong linkage is preferred, the present invention encompasses the possibility that although a covalent linkage is formed, the covalent linkage may not be strong and may, for example, be sensitive to disruption with the application of heat. However, as long as the glass coupling moiety produces the desirable effect of promoting adhesion, the covalent linkage is sufficient. If necessary, catalysts may be used to promote linkage formation.
Slip agent moiety of the composite oligomer do not substantially affect the adhesion of the inner primary coating to the surface of the optical glass fiber. Instead, the slip agent moiety is intended to reduce the sliding force of the inner primary coating against the surface of the optical glass fiber, once the bonds between the surface of the optical glass fiber and inner primary coating are broken (i.e. after the inner primary coating has been delaminated).
Slip agents are known in the art as, among other things, release, antiblocking, antistick, and parting agents. Slip agents are commonly oligomeric or polymeric and are usually hydrophobic in nature, with the most common examples including silicones (or polysiloxanes), fluoropolymers, and polyolefins. The slip agent moiety of the present invention may be derived from such conventional slip agents and may include, for example, silicones, fluoropolymers, and/or polyolefins in combination with polyesters, polyethers and polycarbonates. Additional suitable slip agents are disclosed in, for example, the article entitled xe2x80x9cRelease Agentsxe2x80x9d published in the Encyclopedia of Polymer Science, 2nd Ed., Vol. 14, Wiley-Interscience, 1988, pgs. 411-421, the complete disclosure of which is hereby incorporated by reference. Although slip agents operate over a wide variety of interfaces, the present invention is particularly concerned with an interface of a glass surface, and in particular, the inorganic-organic coating interface between the inner primary coating and the surface of the optical glass fiber. A typical slip agent moiety of the composite oligomer is derived from one of the aforementioned slip agents that is covalently incorporated into the composite oligomer.
In a preferred embodiment, the slip agent moiety is the principal component of the oligomer in terms of weight percent because the slip agent moiety itself is usually oligomeric in nature, and the glass coupling and radiation-curable moieties are usually of lower molecular weight. For example, the slip moiety can be up to about 95 wt. %, relative to the total weight of the composite oligomer, when the three moieties are directly linked together. However, when an oligomeric backbone is present, the slip agent usually can be up to about 85 wt. %, relative to the total weight of the composite oligomer. As with the molecular weight of the composite oligomer of the present invention, the molecular weight of the slip agent moiety is not strictly limited, but will generally be between about 150 and about 9,500, preferably, between about 400 and about 4500.
As with the molecular architecture of the oligomer, there is no particular limitation on the molecular architecture of the slip agent moiety, although in general, substantially linear structures can be used. Non-linear or branched structures, however, are not excluded. Oligomeric slip agent moieties, when present, may contain different kinds of repeat units, although preferably, there is one main type of repeat unit.
Oligomeric silicone slip agent moieties are preferred, and oligomeric silicones comprising substantial portions of methyl side groups are particularly preferred. The side groups preferably impart hydrophobic character to the silicone. Other preferred side groups include ethyl, propyl, phenyl, ethoxy, or propoxy. In particular, dimethylsiloxane repeat units represented by the formula, xe2x80x9cxe2x80x94OSi(CH3)2xe2x80x94xe2x80x9d are preferred.
In a preferred embodiment, the end groups on a substantially linear silicone oligomer can be linked with a radiation-curable moiety at one end and a slip agent moiety at the other end. Such linkage can involve intermediate linkage groups. Although linkage at the silicone oligomer end group is preferred, the silicone moiety can be tailored for linkage with slip agent and radiation-curable moieties at other points in the oligomer molecule besides the end groups. For example, functional groups can be incorporated throughout the molecular structure of the silicone oligomer that are linked with the radiation-curable and slip agent moieties. Examples of functionalized silicones which can be incorporated into the oligomer include polyether, polyester, urethane, amino, and hydroxyl.
Other suitable types of slip agent moieties include those derived from fluorinated slip agents. Examples of such fluorinated slip agents include FC-430, FX-13, and FX-189 (Minnesota Mining and Manufacturing), Fluorolink E (Ausimont), and EM-6 (Elf Atochem).
Generally, the composite oligomer of the present invention is surface active because of the glass coupling moieties, and in particular, may tend to concentrate at coating interfaces, such as the inorganic-organic interface, if not bound in the inner primary coating. However, the covalent binding of the composite oligomer after cure, due to the radiation-curable moiety, may retard such surface activity or migration. Surface activity means that the composite oligomer, when placed in a formulation, tends to migrate to the surface of the formulation rather than be dispersed evenly throughout the formulation.
The radiation-curable moiety should help ensure that the composite oligomer is covalently linked within a radiation-curable coating so that the composite oligomer cannot be extracted or volatilized from the cured coating without breaking covalent bonds.
The radiation-curable moiety can include any functional group capable of polymerizing under the influence of, for example, ultraviolet or electron-beam radiation. One type of radiation-curable functionality is, for example, an ethylenic unsaturation, which in general is polymerized through radical polymerization, but can also be polymerized through cationic polymerization. Examples of suitable ethylenic unsaturation are groups containing acrylate, methacrylate, styrene, vinylether, vinyl ester, N-substituted acrylamide, N-vinyl amide, maleate esters and fumarate esters. Preferably, the ethylenic unsaturation is provided by a group containing acrylate, methacrylate or styrene functionality. Most preferably, the ethylenic unsaturation is provided by a group containing acrylate functionality.
Another type of functionality generally used is provided by, for example, epoxy groups, or thiol-ene or amine-ene systems. Epoxy groups, in general, can be polymerized through cationic polymerization, whereas the thiol-ene and amine-ene systems are usually polymerized through radical polymerization. The epoxy groups can be, for example, homopolymerized. In the thiol-ene and amine-ene systems, for example, polymerization can occur between a group containing allylic unsaturation and a group containing a tertiary amine or thiol.
The amount or number of glass coupling, slip agent, and radiation curable moieties in the composite oligomer is not particularly limited provided that advantages of the present invention can be achieved and the inventive concept is practiced. Thus, a single molecule of the composite oligomer can contain multiple numbers of glass coupling, slip agent, or radiation-curable moieties, although in a preferred embodiment, a single oligomeric molecule contains one glass coupling, one slip agent, and one radiation-curable moiety.
The glass coupling, slip agent, and radiation curable moieties should be covalently linked together in the oligomer. There is no particular limitation to how this linkage is effected provided that advantages of the present invention are achieved and the inventive concept practiced. Linkage may entail direct linkage to the oligomer, or alternatively, indirect linkage to the oligomer. Intermediate linking groups will generally operate by way of two functional groups on a linking compound which can link, for example, the radiation-curable moiety with the slip agent moiety, or link the glass coupling moiety with the slip agent moiety.
Representative linking compounds include diisocyanate compounds, wherein linkage occurs by formation of urethane, thiourethane, or urea links by reaction of, hydroxyl, thiol, and amino groups respectively, with isocyanate. Such diisocyanate compounds are well-known in the polyurethane and radiation-curable coating arts. Aromatic or aliphatic diisocyanates can be used, although aliphatic diisocyanates are preferred. Other linkages can be through, for example, carbonate, ether and ester groups. Preferably, urethane, urea or thiourethane groups are used as the linking groups.
The oligomer, therefore, preferably comprises within its structure at least one linkage. represented by
xe2x80x83xe2x80x94NHxe2x80x94COxe2x80x94Xxe2x80x94
wherein X is an oxygen, sulfur, or nitrogen atom. Urethane and thiourethane groups are most preferred. Urethane groups, for example, can hydrogen bond.
Although the present invention is not limited to one particular molecular architecture for the composite oligomer, in a preferred embodiment which makes use of intermediate linking groups, the composite oligomer can be represented by the following generic structure:
Rxe2x80x94L1xe2x80x94Axe2x80x94L2xe2x80x94C
wherein A represents the slip agent moiety,
R represents a radiation-curable moiety,
C represents the glass coupling moiety, and
L1 and L2 represent linking groups.
L1 and L2 can be independently any group capable of providing a covalent link between the xe2x80x9cRxe2x80x9d moiety and the xe2x80x9cAxe2x80x9d moiety or between the xe2x80x9cCxe2x80x9d moiety and the xe2x80x9cAxe2x80x9d moiety. Based on the disclosure provided herein, one skilled in the art will easily be able to understand what linking groups are suitable for the particular xe2x80x9cAxe2x80x9d, xe2x80x9cCxe2x80x9d and xe2x80x9cRxe2x80x9d groups selected.
In particular, urethane and thiourethane groups are preferred. Urethane and thiourethane linking groups are formed by, for example, (i) linking a hydroxyl end-capped oligomer with a low molecular weight diisocyanate compound at both oligomer ends without extensive coupling of the oligomer, (ii) linking the isocyanate endapped oligomer with a low molecular weight hydroxyacrylate compound, or (iii) linking the isocyanate end-capped oligomer with a low molecular weight mercapto compound.
The linking groups, however, are considered optional. In other words, the oligomer also can be represented by the following generic structures:
Rxe2x80x94L1xe2x80x94Axe2x80x94C,
Rxe2x80x94Axe2x80x94L2xe2x80x94C,
or
Rxe2x80x94Axe2x80x94C.
Although the present invention is disclosed in terms of the aforementioned groups or moieties, other groups can in principle he incorporated into the molecular structure to the extent that the advantages of the present invention can be achieved and the inventive concept practiced.
A preferred embodiment of the present invention is the preparation of a composite oligomer with use of the following ingredients: a silicone oligomer having two hydroxyl end groups (slip agent moiety), isophorone diisocyanate (linkage), hydroxyethyl acrylate (radiation-curable moiety), and mercaptopropyl silane (glass coupling moiety), isophorone diisocyanate (IPDI) serves to end-cap both ends of the silicone diol oligomer and provide a linking site with the hydroxyethyl acrylate at one end of the silicone oligomer and with the mercaptopropyl silane at the other end.
A preferred application for the composite oligomer is as an oligomeric additive, or even as a main oligomeric component, in a radiation-curable coating, and in particular, an inner primary, optical glass fiber coating. The amount of oligomeric additive incorporated into the radiation curable matrix is not particularly limited but will be sufficient or effective to achieve the specific performance objectives of the particular application. In general, however, a suitable amount will be between about 0.5 wt. % and about 90 wt. %, preferably, between about 0.5 wt. % and about 60 wt. %, and more preferably, between about 0.5 wt. % and about 30 wt. % with respect to the total weight of the radiation-curable coating formulation. In general, higher molecular weight composite oligomers will be present in a radiation-curable coating in greater weight percentages than lower molecular weight composite oligomers.
The composite oligomer functions to tailor the properties of formulations which exhibit too great a coefficient of friction or too low adhesion. Specifically, the composite oligomer can increase the adhesion if the adhesion is unacceptably low, and. in particular unacceptably low in the presence of moisture. Alternatively, the composite oligomer can reduce the coefficient of friction of a coating. Conventional coupling additives and slip agents cannot perform this dual function.
If desired, although a reduction in the number of additives is desirable, the composite oligomer can be used in conjunction with other coupling and slip agents to improve absolute performance or cost-performance. In a preferred embodiment, for example, the composite oligomer can be used in conjunction with a functional organosilane compound such as, for example, mercaptopropyl silane. For example, a hydroxybutylvinylether adduct with OCNxe2x80x94(CH2)3Si(OCH3)3 can also be used together with the composite oligomer.
The composite oligomer can be incorporated into a wide variety of radiation-curable formulations. There are no particular limitations provided that the inventive concept is practiced and advantages accrue. One skilled in the art of formulating radiation-curable coatings will easily be able to incorporate the composite oligomer therein to provide the desired properties.
In optical glass fiber coating applications, for example, other formulation components generally include:
(i) at least one multi-functional radiation-curable oligomer, which is a different oligomer than the composite oligomer of the present invention, to provide a cross-linked coating;
(ii) at least one reactive diluent to adjust the viscosity to a level acceptable for application to optical glass fibers, and
(iii) at least one photoinitiator.
Additives such as antioxidants, and as already noted, coupling and slip agents may also be utilized.
Radiation-curing is generally rapidly effected with use of ultraviolet light, although the present invention is not so limited, and a person of skill in the art can determine the best cure method. Radiation-curing results in polymerization of at least some of the radiation-curable moieties present in the composite oligomer which covalently links the composite oligomer to itself or, more preferably, other radiation-curable components in the formulation. The chemical processes which occur upon mixing and curing formulations are in some cases complex and may not be fully understood. The present invention, however, is not limited by theory and can be readily understood and practiced by persons of skill in the art. The formulations of the present invention, just like the composite oligomer, can be in pre-cured, partially cured, and in cured states.
The composite oligomer can be incorporated into inner primary coating compositions, outer primary coating compositions, ink compositions and matrix forming compositions. The composite oligomer also can be incorporated into so-called single coating systems.
In general, the coating substrate, which includes optical fiber, will be an inorganic or glass substrate, although in principle, other substrates such as polymeric substrates may also be effectively used. The glass coupling moiety of the oligomeric additive preferably has the capacity to couple the substrate. In a preferred application, the coating substrate is an optical glass fiber, and in particular, a freshly drawn, pristine optical glass fiber. Freshly prepared optical glass fiber is known in the art to be responsive to glass coupling agents. Exemplary methods of coating optical fibers are disclosed in, for example, U.S. Pat. Nos. 4,474,830 and 4,913,859, the complete disclosures of which are hereby incorporated by reference.
The present inventions will be further explained by use of the following non-limiting examples.